Altimeters measure the altitude of an aircraft. The altimeter actually resides on the aircraft and a block diagram of an altimeter is shown in FIG. 1. In the operation of an altimeter 2, a pulse signal is typically transmitted at a rate of 1 pulse/100 .mu.sec from the aircraft and a return signal is detected by an IF strip 4 in the altimeter 2 upon hitting the nearest point. FIG. 2 shows the return signal and the detection of the nearest point is designated by the leading edge of the return pulse. The return signal is amplified by an amplifier 6 in the altimeter 2 which is then processed by an altimeter processor 8 to determine the distance between the aircraft and the nearest point which is the altitude. The altitude is attained by determining the time between when the signal was sent and when the signal returned and multiplying the difference in time by the speed of light. Once the altitude is determined, the altitude is sent as a signal through a filter 10 to filter out noise and then the output is transmitted from the altimeter 2.
In actuality, the altimeter 2 only measures the distance between the aircraft and the nearest point rather than the actual altitude to the ground. For example, if a plane was flying over a forest, the altimeter would only measure the distance to the trees and not the ground. FIG. 3 shows what a return signal with trees might reveal. The first "edge" would represent the detection of the trees and the second "edge" would represent the ground.
In present radar altimeters, they are only capable of measuring the distance to the nearest point. The present altimeter can only measure the distance to the nearest point due to the processing within the altimeter processor 8. The altimeter processor 8 includes an integrator within a closed loop servo designed to control the position of a moveable radar range gate such that the gate encompasses only the first leading edge which is from the nearest point and not the second edge which is from the ground. Looking at FIG. 3, the shaded portion 12 designates the amount of the signal the gate passes while the rest of the signal is lost. As can be seen, a very small portion 12 of the entire signal is actually processed and the rest of the signal including the second "edge", which designates the ground, is never detected. Servo loops are well known in this area of technology and will not be discussed in any further detail here.
Before, there was only interest in determining the distance to the nearest point for safety issues in order to prevent the aircraft from crashing into the nearest point whether it was trees or the ground. As a result, the rest of the signal was not necessary for processing and the altimeter 2 was adequate for past purposes. However, presently, it is now necessary to determine the actual altitude of the aircraft as well as the nearest point to the aircraft. Therefore, it would be beneficial if an altimeter had the capability to determine the altitude of an aircraft as well as the distance from the aircraft and the nearest point for safety measures.